Chapter 10: Halogen Derivatives Mock Test
Time: 1 Hour | Maximum Marks: 25
- All questions are compulsory.
- Section A contains Q1 (Multiple Choice) and Q2 (Very Short Answer).
- Section B contains Short Answer Type I questions (2 marks each). Attempt any 4.
- Section C contains Short Answer Type II questions (3 marks each). Attempt any 2.
- Section D contains Long Answer questions (4 marks each). Attempt any 1.
SECTION A
Q1. Select and write the most appropriate answer from the given alternatives: [4 Marks]
-
The $S_N2$ mechanism of an alkyl halide with aqueous KOH proceeds with:
(A) 100% Inversion of configuration(B) 100% Retention of configuration(C) Racemization(D) Rearrangement
-
Which of the following reagents is used in the Swarts reaction to prepare alkyl fluorides?
(A) $NaI$ in dry acetone(B) $AgF$ or $CoF_3$(C) $SOCl_2$(D) $PCl_5$
-
When ethyl bromide is treated with metallic sodium in dry ether, the major product formed is:
(A) Methane(B) Ethane(C) Butane(D) Propane
-
For a molecule to be optically active, it must possess:
(A) A plane of symmetry(B) A center of symmetry(C) At least one chiral carbon atom(D) An even number of carbon atoms
Q2. Answer the following questions in one sentence: [3 Marks]
- Identify the best reagent for converting an alcohol into an alkyl chloride without the need to separate by-products.
- Define: Racemic mixture.
- What is the major product obtained when 2-bromobutane is heated with alcoholic KOH?
SECTION B
Attempt any FOUR of the following: [8 Marks]
- Distinguish between Enantiomers and Diastereomers.
- Haloarenes are much less reactive towards nucleophilic substitution reactions compared to haloalkanes. Give two reasons.
- Explain the Finkelstein reaction with a suitable chemical equation.
- State Markovnikov's rule and illustrate it with a suitable example.
- Why must Grignard reagents be prepared under strictly anhydrous conditions? Explain with a reaction.
SECTION C
Attempt any TWO of the following: [6 Marks]
- Describe the $S_N2$ mechanism for the alkaline hydrolysis of methyl bromide.
- Differentiate between $S_N1$ and $S_N2$ mechanisms. (Write any 3 points of distinction).
- Explain the Wurtz-Fittig reaction with a chemical equation. How does it differ from the Fittig reaction?
SECTION D
Attempt any ONE of the following: [4 Marks]
- (a) Describe the $S_N1$ mechanism for the alkaline hydrolysis of tert-butyl bromide. [3 Marks]
(b) What is an ambident nucleophile? Give one example. [1 Mark] - (a) State Saytzeff's rule. Illustrate it with the dehydrohalogenation reaction of 2-bromobutane. [3 Marks]
(b) Define Optical Activity. [1 Mark]
Solutions & Marking Scheme
SECTION A [7 Marks]
Q1. Multiple Choice Answers:
1. (A) 100% Inversion of configuration [1 Mark for correct option]
2. (B) $AgF$ or $CoF_3$ [1 Mark for correct option]
3. (C) Butane [1 Mark. Wurtz reaction: $2C_2H_5Br + 2Na \rightarrow C_4H_{10}$]
4. (C) At least one chiral carbon atom [1 Mark for correct option]
Q2. Very Short Answers:
1. Best Reagent (Darzen's Method):
Thionyl chloride ($SOCl_2$). The byproducts ($SO_2$ and $HCl$) are gases that escape, leaving pure alkyl chloride. [1 Mark]
2. Racemic Mixture:
An equimolar mixture of two enantiomers (dextro and laevo forms) which is optically inactive due to external compensation is called a racemic mixture. [1 Mark]
3. Product of 2-bromobutane with alc. KOH:
The major product is But-2-ene (according to Saytzeff's rule). [1 Mark]
SECTION B [8 Marks]
Q3. Enantiomers vs Diastereomers:
| Enantiomers | Diastereomers |
|---|---|
| They are stereoisomers that are non-superimposable mirror images of each other. | They are stereoisomers that are not mirror images of each other. |
| They have identical physical properties (except rotation of polarized light). | They have different physical properties (e.g., MP, BP, density). |
[1 Mark for each point of distinction. Total 2 Marks]
Q4. Reactivity of Haloarenes:
- Resonance Effect: The lone pair on the halogen is in conjugation with the $\pi$-electrons of the benzene ring, giving the C-X bond a partial double bond character. It is difficult to break. [1 Mark]
- Hybridization: The halogen is attached to an $sp^2$ hybridized carbon, which is more electronegative and holds the electron pair more tightly than the $sp^3$ carbon in haloalkanes. [1 Mark]
Q5. Finkelstein Reaction:
Alkyl iodides are prepared by treating alkyl chlorides or bromides with Sodium Iodide ($NaI$) in the presence of dry acetone. This halogen exchange reaction is known as the Finkelstein reaction. [1 Mark]
$R-X + NaI \xrightarrow{\text{Dry Acetone}} R-I + NaX \downarrow$ (where X = Cl, Br) [1 Mark]
Q6. Markovnikov's Rule:
Statement: When an unsymmetrical reagent is added to an unsymmetrical alkene, the negative part of the reagent attaches to the carbon atom of the double bond that carries fewer hydrogen atoms. [1 Mark]
Example: Addition of HBr to Propene yields 2-Bromopropane as the major product.
$CH_3-CH=CH_2 + HBr \rightarrow CH_3-CH(Br)-CH_3$ [1 Mark]
Q7. Grignard Reagent Condition:
Grignard reagents ($R-Mg-X$) are highly reactive and act as strong bases. If moisture (water) is present, the Grignard reagent immediately reacts with the active hydrogen of water to form an alkane, destroying the reagent. [1 Mark]
$R-Mg-X + H_2O \rightarrow R-H \text{ (Alkane)} + Mg(OH)X$ [1 Mark]
SECTION C [6 Marks]
Q8. $S_N2$ Mechanism of Methyl Bromide:
Reaction: $CH_3-Br + OH^- \rightarrow CH_3-OH + Br^-$
Kinetics: It follows second-order kinetics. $\text{Rate} \propto [CH_3Br][OH^-]$. [1 Mark]
Mechanism: It is a single-step concerted mechanism. The nucleophile ($OH^-$) attacks the carbon from the backside (opposite to the leaving group, $Br^-$) to avoid steric hindrance. [1 Mark]
It forms a transition state where C is partially bonded to both OH and Br. As the reaction completes, it results in 100% Inversion of configuration (Walden Inversion). [1 Mark]
Q9. $S_N1$ vs $S_N2$ (Any 3 points):
| $S_N1$ Mechanism | $S_N2$ Mechanism |
|---|---|
| 1. It is a two-step mechanism. | 1. It is a single-step concerted mechanism. |
| 2. Forms a carbocation intermediate. | 2. Forms a transition state (no intermediate). |
| 3. Kinetics: First order ($\text{Rate} \propto [RX]$). | 3. Kinetics: Second order ($\text{Rate} \propto [RX][Nu^-]$). |
| 4. Leads to Racemization. | 4. Leads to complete Inversion of configuration. |
[1 Mark for each point of distinction. Total 3 Marks]
Q10. Wurtz-Fittig and Fittig Reactions:
Wurtz-Fittig Reaction: A mixture of an alkyl halide and an aryl halide reacts with sodium in dry ether to form an alkylbenzene. [1 Mark]
$C_6H_5-Br + 2Na + Br-CH_3 \xrightarrow{\text{Dry Ether}} C_6H_5-CH_3 \text{ (Toluene)} + 2NaBr$ [1 Mark]
Difference from Fittig: The Fittig reaction involves the coupling of two aryl halides to form a diaryl (like biphenyl), whereas Wurtz-Fittig couples one alkyl and one aryl halide. [1 Mark]
SECTION D [4 Marks]
Q11. (a) $S_N1$ Mechanism [3 Marks] (b) Ambident Nucleophile [1 Mark]
(a) $S_N1$ Mechanism of tert-butyl bromide:
Reaction: $(CH_3)_3C-Br + OH^- \rightarrow (CH_3)_3C-OH + Br^-$
The rate depends only on $[(CH_3)_3C-Br]$. It occurs in two steps: [1 Mark]
Step 1 (Slow/Rate-determining): Heterolytic cleavage of C-Br bond to form a planar, stable tertiary carbocation.
$(CH_3)_3C-Br \xrightarrow{\text{slow}} (CH_3)_3C^+ + Br^-$ [1 Mark]
Step 2 (Fast): The nucleophile ($OH^-$) can attack the planar carbocation from either the front or the back with equal probability, leading to a racemic mixture (retention and inversion).
$(CH_3)_3C^+ + OH^- \xrightarrow{\text{fast}} (CH_3)_3C-OH$ [1 Mark]
(b) Ambident Nucleophile: A nucleophile that has two different donor atoms and can attack through either of them. Example: Cyanide ion ($CN^-$ can attack through C or N). [1 Mark]
Q12. (a) Saytzeff's Rule [3 Marks] (b) Optical Activity [1 Mark]
(a) Saytzeff's Rule:
Statement: During dehydrohalogenation, if more than one alkene can be formed, the major product is the alkene that is more highly substituted (has more alkyl groups attached to the double-bonded carbons). [1 Mark]
Illustration: When 2-bromobutane is heated with alcoholic KOH, $\beta$-elimination occurs.
$CH_3-CH_2-CH(Br)-CH_3 \xrightarrow{\text{alc. KOH}}$
1. $CH_3-CH=CH-CH_3$ (But-2-ene) [Major product, 80%]
2. $CH_3-CH_2-CH=CH_2$ (But-1-ene) [Minor product, 20%]
(b) Optical Activity: The property of certain organic substances containing chiral molecules to rotate the plane of plane-polarized light (either clockwise or anti-clockwise). [1 Mark]
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